Landing gear for most commercial aircraft include a main shock strut having one end pivotably attached to a portion of the airplane for swinging movement of the landing gear between a retracted and extended position. The landing gear also includes a truck beam pivotably attached to the other end of the main shock strut. A plurality of wheels are journaled to the truck beam. It is desirable for the truck beam to be pivotably attached to the main shock strut to permit the beam to pivot and absorb energy associated with traversing a bump or a rough runway, as well as permitting the truck beam to be positioned for stowage within the airplane. Extending between the main shock strut and the truck beam is a hydraulic truck positioner. The truck positioner is adapted to position the truck beam for stowage within the airplane.
FIG. 1 illustrates a currently available truck positioner 10. The truck positioner 10 includes a cylindrical housing 12, a floating piston 14, a main piston 16 and a hydraulic system 18. For ease of illustration, the hydraulic system 18 is illustrated as a schematic. The main piston 16 includes a piston head end slidably received within the housing 12. The other end of the main piston 16 is pinned to the truck beam to enable positive positioning of the truck beam after take off, thereby ensuring safe retraction of the landing gear by positioning the wheels clear of adjacent wheel well structure and equipment during flight. The floating piston 14 allows extra extension of the actuator when hydraulic pressure is off, which is required for changing tires.
Thus, currently available truck positioners are hydraulic actuators designed to selectively pivot the truck beam about its pivot joint to position the truck beam into a desired predetermined stowage position.
Although currently available truck positioners are effective at positioning the truck beam for stowage within the aircraft, they are not designed to dampen loads associated with a pivotable truck beam. As noted above, the truck beam is designed to pivot about its pivot attachment joint. When traveling on rough runways, the truck beam becomes excited at its natural frequency, causing high frequency pivoting of the pivot beam. Such high frequency pivots result in premature failure of the pivot joint. In particular, investigation of failed pivot joints has indicated that such failures are caused by aggressive pivot pin rotation linked to rapid truck beam oscillations characteristic of rough runway operations. Further, certain airlines typically load their airplanes to their maximum capacity, thereby aggravating the adverse effects of rough runways on the highly loaded pivot joint.
Under such operating conditions, high frequency oscillations about the pivot joint causes the joint to overheat, adversely affecting the material's properties. As a result, such assemblies become hard and brittle and, therefore, susceptible to crack generation. Ultimately, such cracks cause premature failure of the pivot joint. Thus, there exists a need for a damping assembly to minimize the displacements of high frequency oscillations of a landing gear truck beam.